Seismic waves generated artificially for the imaging of geological layers has been used for more than 50 years. The most widely used waves are by far reflected waves and more precisely reflected compressional waves. During seismic prospection operations, vibrator equipment (also known as a “source”) generates a vibro-seismic signal that propagates in particular in the form of a wave that is reflected on interfaces of geological layers. These waves are received by geophones, or receivers, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by means of recording equipment. Analysis of the arrival times and amplitudes of these waves makes it possible to construct a representation of the geological layers on which the waves are reflected.
Sweep design pertains to the choice of frequencies used to drive the sound producing device used for determining the possible or probable location of hydrocarbon deposits under, e.g., land surface or the ocean floor. The sound producing device in such an application can be referred to as a vibrator, and is generally also called the “source,” i.e., the source of the sound waves that are transmitted and then reflected by various geological interfaces and then received by one or more, usually dozens, of receivers. In one particular method of use, the sources are referred to as vibrators when used off shore, for exploration under the ocean floor.
FIG. 1 depicts schematically a device for transmitting and receiving vibro-seismic waves intended for seismic exploration in a land environment. The device comprises a source consisting of a vibrator 1 operable to generate a seismic signal, a set of receivers 2 (or geophones) for receiving a seismic signal and converting it into an electrical signal and a recorder 3 for recording the electrical signals generated by the receivers. The source 1, the receivers 2 and the recorder 3 are positioned on the surface of the ground 5. FIG. 1 depicts a single vibrator but it should be understood that the source may be composed of several vibrators, as is well known to persons skilled in the art.
In operation, source 1 is operated so as to generate a vibro-seismic signal. This signal propagates firstly on the surface of the ground, in the form of surface waves 4, and secondly in the subsoil, in the form of waves 6 that generate reflected waves when they reach an interface 7 between two geological layers. Each receiver 2 receives both a surface wave 4 and a reflected wave 6 and converts them into an electrical signal in which are superimposed the component corresponding to the reflected wave and the one that corresponds to the surface wave, which is undesirable and which is to be filtered.
One class of sources that can be used is vibratory sources. Vibratory sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in seismic operations. A vibrator generates a long tone with a varying frequency, i.e., a frequency sweep. This signal is applied to a moving part, e.g., a piston, which generates a corresponding seismic wave. Instantaneous pressure resulting from the movement of plural pistons corresponding to plural vibrators may be lower than that of an airgun array, but total acoustic energy transmitted by the vibrator may be similar to the energy of the airgun array due to the extended duration of the signal. However, such sources need a frequency sweep to achieve the required energy. The design of the frequency sweep will now be discussed.
A sweep is a sinusoid with a continuously variable frequency, and can be defined by its amplitude A(f) and its sweep rate Sr(f) defined as the derivative of the frequency relative to time df/dt. Provided the sweep is long enough (longer than 5 or 6 seconds), the amplitude spectrum of the sweep at frequency f is proportional to A(f) and to the square root of 1/Sr(f). Target-oriented sweep design (i.e., searching for a particular known type of hydrocarbon, in a particular known type of geological formation) consists in defining A(f) and Sr(f) to obtain the desired Signal-to-Noise ratio (SNR) of the target reflection.
Non-linear sweeps were introduced in late 1970s. At that time, the purpose of these sweeps was to generate a higher proportion of high frequencies that are attenuated by non-elastic wave propagation. The limited flexibility of the available software allowed the choice between a very small amplification, which did not significantly differ from conventional linear sweeps, and larger amplifications, which being constant over the entire frequency range often resulted in a damaging reduction in the low frequency content. This drawback was noticed and more sophisticated electronics were developed to allow more flexibility. An example of such a technique contributing to an enhancement was shown by D. Mougenot in 2002, as shown in FIG. 2. FIG. 2 illustrates a comparison of conventional (line A) and segmented logarithmic sweeps (line B). It can be readily observed in FIG. 2 that line A represents a linear rate of change of the sweep frequency, beginning at about 10 Hz, and changing linearly to about 120 Hz, over a 20 second period of time. Note that the relative amplitude of the transmitted signal also changes over time, from about 10-15 dB to about 55 dB by the end of the sweep period. The sweep proposed by Mougenot and Meunier in 2002 began at about 10 Hz, and proceeds to about 43 Hz over a two second period of time, but at about a constant (or linear) gain of 35 dB. Then, the sweep frequency changes from about 43 Hz to about 71 Hz over a two second period of time, and changes in amplitude from about 35 dB to about 37-38 dB. Finally, in the last phase, the sweep frequency changes from about 71 Hz at about 37-38 dB to 133 Hz and just over 60 dB in a 26 second time period. With such a change in the amplitude and frequencies over time, some gains were realized in maintaining low frequency content, without sacrificing the higher frequency affects. However, even with the realized benefits, there remained problems with the sweep frequencies.
In 2007, C. Bagaini, in a paper entitled “Enhancing the Low-Frequency Content of Vibroseis Data with Maximum Displacement Sweeps,” the entire contents of which are hereby incorporated by reference, and which was presented at the 69th EAGE Conference & Exhibition, discussed modifying the sweep rate of land vibrators to compensate for stroke limitations in the low frequencies. As those of ordinary skill in the art can appreciate, “stroke” pertains to the movement of an oscillating piston in a vibrator that produces the sound wave underwater. An extension of this approach was proposed by J. Sallas in 2009, in U.S. Published Patent Application No. 2011-0085416, entitled “System and Method for Determining a Frequency Sweep for Seismic Analysis,” the entire contents of which are hereby incorporated by reference. Low frequency limitations are not restricted to land seismic. The difficulty of moving the reaction mass along a longer distance to obtain low frequencies from a land vibrator is very similar to the difficulty of moving a larger volume of water to obtain low frequencies from a marine vibrator.
A sweep design method for a seismic land vibrator is also disclosed in U.S. Pat. No. 7,327,633, entitled, “Systems and methods for enhancing low-frequency content in Vibroseis acquisition,” the entire content of which is incorporated herein. The patent discloses a method for optimizing sweep signal strength by taking into account a single physical property of a seismic land vibrator, i.e., a stroke limit of the seismic vibrator device. A non-linear sweep is obtained in order to build up the sweep spectral density to achieve a targeted spectrum in the low frequency range. However, other physical properties of the seismic land vibrator, which limit the operation of the land vibrator, are not considered. Further, this patent is directed to a land vibrator, which is different from a marine vibrator.
A more sophisticated sweep design method is disclosed in U.S. patent application Ser. No. 12/576,804, entitled, “System and method for determining a frequency sweep for seismic analysis,” the entire content of which is incorporated herein by reference. This method takes into account not only the plate stroke limit but also other constraints of the land vibrator, e.g., the pump flow limit and the servo valve flow limit. However, this method addresses a land vibrator, which has different characteristics than a marine vibrator, and the method also does not take into consideration specific features of the water environment.
Accordingly, it would be desirable to provide methods, modes and systems for the design of sweep that takes into account constraints of the vibrator and, optionally, constraints imposed by the water environment.